The present invention relates to an ion guide, a mass spectrometer, a method of guiding ions and a method of mass spectrometry. The preferred embodiment relates to an ion guide or ion transport device which preferably uses a combination of a DC voltage and an AC or RF voltage in order to focus and/or transport ions through the ion guide or ion transport device preferably in the presence of background gas.
A known multipole rod set ion guide comprises four, six or eight parallel rods which are equi-spaced about a circular circumference. Opposite phases of a two-phase RF voltage are applied to adjacent rods. The RF voltage applied to the rods generates a symmetrical pseudo-potential well within the ion guide which acts to confine ions radially within the ion guide. If the ion guide is operated at a relatively high pressure then the ion radial density distribution may also be reduced due to the effect of collisional cooling wherein ions lose kinetic energy after colliding with gas molecules.
Another known ion guide comprises a plurality of ring electrodes having apertures through which ions are transmitted. Opposite phases of a two-phase RF voltage are applied to adjacent ring electrodes. The ion guide may comprise an ion tunnel ion guide comprising ring electrodes which all have substantially the same diameter apertures. Alternatively, the ion guide may comprise an ion funnel ion guide comprising ring electrodes having apertures which progressively reduce in diameter along the axial length of the ion guide.
Another known ion guide comprises a stack or array of layers of intermediate electrodes which are arranged horizontally in the plane of ion motion. Each intermediate layer comprises two longitudinal electrodes which are spaced apart from one another with an ion guiding region provided in between. Opposite phases of an RF voltage are applied to vertically adjacent or neighbouring layers of intermediate electrodes. The two longitudinal electrodes in any of the layers of intermediate electrodes are connected to the same phase of the RF voltage. The ion guide also further comprises an upper planar electrode and a lower planar electrode which act to confine ions in the vertical radial direction. A DC and/or AC or RF voltage may be applied to the upper and lower planar electrodes in order to confine ions within the ion guide.
The known multipole rod set ion guide provides ion confinement in the radial direction when used to transmit a relatively narrow beam of ions. However, it is problematic to increase the size of the ion guide in the radial dimension in order to capture ions from a more diffuse source since this requires increasing the RF voltage applied to the rods in proportion to the square of the radius. Furthermore, even with the same confining effective potential barrier the degree of focussing would be reduced in a larger ion guide due to the reduced radial effective potential gradient.
It may also be problematic to attempt to use an ion tunnel ion guide in conjunction with a diffuse ion source.
Although an ion funnel ion guide may be used to focus ions from a diffuse source, there is a direct line of sight between the ion entrance aperture and the ion exit aperture. The same is also true of an ion guide comprising a stock or array of planar electrodes arranged in the plane of ion motion. Such ion guides can suffer from the problem of gas streaming which increases the pumping requirements. Furthermore, if a mixture of gas and ions is arranged to enter the ion guide and the mixture also contains neutral species or droplets then these can pass through the ion guide and contaminate the various apertures.
It is therefore desired to provide an improved ion guide.